1. Field of the Invention
This invention is related in general to processes for the manufacture of photonic polymer composite films. In particular, the invention pertains to large-area flexible photonic films with self-healing properties produced by flash evaporation, vacuum deposition and radiation curing.
2. Description of the Related Art
Organic light-emitting diodes (OLEDs) consist of the combination of a hole-transport (electron-donor) organic material with an electron-transport (electron-acceptor) compound such as an organometallic substance. Both materials may be in the form of monomers, oligomers or polymers combined in a single layer or in multi-layer composite structures sandwiched between two electrodes. The composites are typically deposited on a carrying substrate either by a solvent-based technique or a vapor-deposition process. For example, U.S. Pat. Nos. 5,902,641 and 6,040,017 describe flash-evaporation techniques for the deposition of single- and multi-layer structures, respectively.
Manufacturing defects in the active layers of OLEDs, such as may be caused by embedded foreign particles, micro-inclusions, or micro-protrusions, can produce electromechanically and dielectrically weak spots that in turn can lead to localized structural breakdowns. These pinhole occurrences typically result in a high localized current flow, or even in the formation of an arc, between the anode and the cathode electrodes of the OLED. When this kind of dielectric breakdown occurs, the capacitive energy stored between the two electrodes is first discharged, and then it is followed by a continuous current flow supplied by the power source energizing the OLED. This type of breakdown and the attendant high currents cause the formation of elemental carbon that further enhances the electrical conductivity between the electrodes in the vicinity of the damaged spot, thereby producing more current and in turn the formation of more conductive carbon. Thus, the damage propagates and, if not controlled, it leads to complete failure and functional destruction of the OLED device. This is a major drawback in the utilization of currently available OLED technology for many potential applications in which it could otherwise be advantageously adopted.
Another problem with present organic photonic systems lies in the rigidity and brittleness of their structure. All small organic molecules used to produce hole-transport layers (HTL) and electron-transport layers (ETL) consist of crystals that form very brittle thin films. Thus, while these materials may be appropriate for small-area glass-supported devices, less brittle and tougher thin films are required for large-area flexible light sources. Accordingly, the possibility of improving the mechanical properties of photonic films by including such active small molecules inside a polymer matrix has been investigated. Because of the poor solubility of photonic molecules in polymeric materials and the tendency of these molecules to separate out of the polymeric matrix, this line of research has not produced satisfactory results.
Therefore, there is still a need for an organic photonic structure that is both self-healing from pin-hole dielectric failures and sufficiently flexible to warrant the manufacture of large-area OLED devices. This invention is directed at a novel process for achieving these objectives through flash evaporation, vapor deposition, and curing of the various constituents under parameters selected to yield a flexible, self-healing, composite photonic layer.
The primary objective of this invention is an organic LED structure that prevents the propagation of the damage caused by the occurrence of an electrical short between the electrodes of the device.
Another important objective is a sufficiently flexible photonic structure to permit large-area applications over flexible substrates.
Another goal is an advantageous method of manufacture for photonic composites that have such self-healing and flexible-structure characteristics.
Yet another goal is a production process suitable for making large-area photonic products by direct deposition of the photonic layer or layers over a flexible substrate.
Another objective is a process that can be implemented at high production rates.
Still another goal is a process that produces a highly homogeneous, defect-free, film.
Specifically, a goal of the invention is a process particularly suitable for manufacturing large light sources for buildings, such as for wall, ceiling, and window signs.
Another specific goal is a process particularly suitable for manufacturing thin-film lights for the automotive industry.
Yet another specific goal is a process suitable for manufacturing flexible photonic composites for general decorative applications.
A final objective is a method of manufacture that can be implemented relatively easily and economically utilizing modified prior-art vapor deposition technology.
Therefore, according to these and other objectives, one aspect of the invention consists of flash evaporating a heterogeneous blend of small photonic organic molecules and a binder consisting of polymerizable monomers or oligomers to provide a vapor-phase mixture at the molecular level. The mixture is then condensed as a homogeneous liquid layer on a substrate and cured in-line within a very short time (in the order of milliseconds) to ensure that phase separation of the homogeneous condensed film does not occur. According to another aspect of the invention, the surface resistance of the metallic cathode and the chemical characteristics of the polymer binder in the photonic structure are selected such as to ensure the melting and oxidation of exposed portions of the cathode and the complete combustion of elemental carbon generated during a dielectric breakdown in the device, thereby providing a built-in mechanism for minimizing conductivity and preventing the propagation of the damage caused by electrical shorts. The resulting characteristics of self-healing and flexibility of the OLED structure are advantageously utilized to produce large-area flexible light sources for automotive, sign, and decorative applications.
Various other purposes and advantages of the invention will become clear from its description in the specification that follows and from the novel features particularly pointed out in the appended claims. Therefore, to the accomplishment of the objectives described above, this invention consists of the features hereinafter illustrated in the drawings, fully described in the detailed description of the preferred embodiment and particularly pointed out in the claims. However, such drawings and description disclose but one of the various ways in which the invention may be practiced.